1. Field of the Invention
The present invention relates to signaling system for railroads, and in particular to an electronic vital relay used to detect the presence of a train or a broken rail in particular track sections and/or track fault conditions between track sections.
2. Description of the Prior Art
Railroad systems, such as electrified railroad systems that employ power frequently track circuits, utilize signaling systems to provide signals for train operators and to control the operation of railroad crossing gates and warning lights. In such systems, each track circuit is bounded by insulated joints so that the presence of a train can be defined to a unique section of track. Railroad signaling systems use what is termed a vital relay or track relay to detect the presence of a train or a broken rail in particular track sections and/or track fault conditions between track sections (i.e., a fault in the insulated joint connecting two track sections). The signaling system only permits trains to pass and/or crossing gate and warning light systems to be in an off condition when the relay is in an operated condition. The relay will drop out of an operated condition if a train or a broken rail is in the particular track section being controlled or if there is otherwise a fault on the system which prevents an accurate detection of the presence of a train.
A typical use of a vital relay in a track circuit is shown in FIG. 1. As seen in FIG. 1, a track section T of a stretch of electrified railroad is shown with its rails 1 and 2 illustrated by conventional single line symbols. The rails of section T are electronically insulated from the rails of the adjoining sections by the insulated joints 3, also illustrated by conventional symbols. In order to provide a return circuit for the propulsion current, impedance bond windings 4 are connected across rails 1 and 2 at each end of section T and the associated ends of the adjoining sections. The center taps of each associated pair of bond windings 4 are connected by a lead 5 to provide a conventional circuit path through section T for propulsion current.
The signaling system for this stretch of railroad is based on continuous train detection using a track circuit for each track section such as section T. Signaling energy for the track circuits is provided from a central source S having a frequency of, for example, 50, 60 Hz or 100 Hz, and is distributed along the stretch of railroad by the line wires 6 and 7. Energy is supplied across the rails of section T at the left or transmitting end through a track transformer 8 from the line wires 6 and 7. Even though AC energy is used, the supply connections are such that the instantaneous polarity of the rails on each side of the insulated joints 3 are opposite, as indicated by the polarity markings at the rails 1 and 2. The supply connections include a selected resistor 11 which limits the current flow when a train shunts the rails 1 and 2 at the transmitting end. At the other or receiving end of section T, a vital relay circuit 12 is connected across the rails 1 and 2 through a track transformer 13 and a control coil 14 (alternatively, they may be directly connected) and to the line wires 6 and 7 through a local coil 15. As discussed above, the vital relay circuit 12 detects the presence of a train or a broken rail and/or an insulated joint track fault condition in the section T, which detection is in turn used by the signaling system to provide signals for train operators.
As will be appreciated, if a train is not present and no rail is broken in the section T, a substantial amount of current will be present in the control coil 14. However, if a train is present in the section T, it will shunt the rails 1 and 2, thereby resulting in little or no current in the control coil 14. Similarly, there will be little or no current in the control coil 14 if a rail is broken in the section T. In addition, if the insulating joints 3 are intact, the current in the control coil 14 and the current in the local coil 15 will be substantially in phase. However, if a fault condition develops at the insulating joints 3, the current in the control coil 14 and the current in the local coil 15 will be out of phase. These principles are utilized by vital relays to detect the presence of trains and fault conditions in track sections.
A common type of vital relay in use in electrified railroad systems is what is known as a vane relay. A vane relay operates by a principle similar to that of a watt-hour meter. A vane relay includes a vane positioned in the magnetic gap between two coils (i.e., the control coil 14 and the local coil 15). The vane is responsive to the product of: (i) the current in the control coil, (ii) the current in the local coil, and (iii) the cosine of the angular difference of the current in the two coils. Maximum torque in a first direction is produced in the vane if current of a certain magnitude is present and the angular difference is zero (cosine of 0°=1), and maximum torque is produced in a second, opposite direction if current of a certain magnitude is present and the angular difference is 180° (cosine of 180°=−1). In addition, as the level of current decreases, the level of torque in either direction will decrease. The vane is fitted with a ladder structure so that a multiplicity of electrical contacts will close only when the vane rotates with sufficient torque in the first direction, which proves that the current in the two windings is nominally in phase and the current is of sufficient magnitude, i.e., there is not a train or a broken rail in the track section and there is no insulated joint fault condition. Just as important for railway train detection purposes, the contacts will not close if the phase relationship is reversed (indicates a fault at insulating joints 3), regardless of the magnitude of the product of current and angular difference, or if the current in one of the windings is not of sufficient magnitude (i.e., substantially zero) (indicates the presence of a train or broken rail).
The problem with vane relays is that, as an electromechanical device with moving parts, they require considerable preventive maintenance to assure reliable operation. In addition, because vane relays are a product based device, the control current required to close the contacts is inversely effected by the local voltage. If local voltage regulation is poor, safety or reliable operation can be impacted. For example, if the local voltage decreases, track current decreases proportionally, but the control current required to maintain the track relay energized is increased. In such a situation, the potential exists for the track relay to drop and falsely indicate that the track circuit is occupied. Alternatively, at increased local voltage, the rail current is greater, but the current that is required to maintain the track relay energized is decreased. This increases the risk of the track relay's failure to drop in the presence of a broken rail.